[PATCH] ufs: validate fs_cgsize when reading the superblock

[Guikun Yang]

fs_cgsize is read from disk into uspi->s_cgsize without any check.
Its only consumer derives a buffer count from it:

	UCPI_UBH(ucpi)->count = uspi->s_cgsize >> sb->s_blocksize_bits;
	...
	for (i = 1; i < UCPI_UBH(ucpi)->count; i++) {
		UCPI_UBH(ucpi)->bh[i] = sb_bread(...);

struct ufs_buffer_head.bh[] has UFS_MAXFRAG (8) entries, so a crafted
fs_cgsize whose block shift exceeds 8 writes past the array while
loading a cylinder group. fs_cgsize of zero, or smaller than the
fragment size, produces a zero count and leaves the cylinder group in
a half-initialized state.

Reject those values where the rest of the geometry is already
validated.

Reported-by: Guikun Yang <yangguikun19@xxxxxx>  (bug report of 2026-10-31, <c5-ufs-cgsize-ubsan-20261031@xxxxxx>)
Signed-off-by: Guikun Yang <yangguikun19@xxxxxx>
---
 fs/ufs/super.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/ufs/super.c b/fs/ufs/super.c
index 1111111..2222222 100644
--- a/fs/ufs/super.c
+++ b/fs/ufs/super.c
@@ -1140,6 +1140,14 @@ static int ufs_fill_super(struct super_block *sb, void *data, int silent)
 	uspi->s_cgsize = fs32_to_cpu(sb, usb1->fs_cgsize);
+	if (!uspi->s_cgsize ||
+	    uspi->s_cgsize < uspi->s_fsize ||
+	    (uspi->s_cgsize >> sb->s_blocksize_bits) > UFS_MAXFRAG) {
+		pr_err("%s(): invalid cylinder group size (%u)\n",
+		       __func__, uspi->s_cgsize);
+		goto failed;
+	}
 	uspi->s_ntrak = fs32_to_cpu(sb, usb1->fs_ntrak);

Tested on the image from the bug report: the crafted fs_cgsize is now
rejected at mount with the message above instead of writing past
bh[]; the unmodified image still mounts.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
--
2.43.0